The present invention relates to a built-in antenna device for use in a small-sized portable radio such as a cellular phone, which is capable of independently adjusting a plurality of frequencies in order to cope with a plurality of frequency bands.
In Japan, a total number of subscribers of both a cellular phone service and a personal handy phone system (PHS) service exceeds 82 million (as of the end of June, 2003), and the mobile phones are generally carried (or common) items since different frequency bands are provided between the cellular phone service and the PHS service, a mobile phone used for both services must be designed to handle the different frequency bands.
Moreover, there has been a rising demand for speeding up data communication in a field of radio communication as well as that of cable communication. There has also been a demand for enhancement of multi-accessability indicating that many terminals can access one base station, and this is a problem inherent in the radio communication. To meet these demands, second generation systems such as PDC (Personal Digital Communications) and GSM (Global System for mobile Communications) in which a time division multiple access (TDMA) is adopted are shifting to third generation systems such as cdma2000 and W-CDMA in which a code division multiple access (CDMA) is adopted. In the shifting, a different frequency band exclusive for a third generation system is allocated, but a service area thereof has spread at a slow pace because of costs. As a realistic solution of this problem, a compound portable (or mobile) terminal that can be used both in the second and third generations has appeared.
Furthermore, in recent years, in consideration of design, portability and the like, portable terminals have increased in which in-built antennas built in housings of them are adopted rather than antennas protruding to the outside. This tendency can also be seen in the above-described compound portable terminal.
In the compound portable terminal, in addition to a plurality of built-in radios in accordance with the corresponding frequency bands, a plurality of antennas are also required. In the compound portable terminal designed assuming that the terminal is carried by a user, it is difficult to build all the antennas in the terminal.
Additionally, when a plurality of antennas are miniaturized and built in the terminal, interference is frequently caused by resonance among the antennas, and this has been a bottleneck in designing a miniature radio. However, the problem of radio interference has been substantially solved by use of a duplexer.
As an example of a prior technique against these problems, Japanese Patent Application Laid-Open No. 2003-087025 has been proposed. That is, a cellular phone has been disclosed in which a first antenna is disposed through the upper surface of a housing, a second antenna is disposed in lower portion of the housing, which is hard to be covered by an operator's hand, and accordingly two antennas are separated from each other to prevent resonance from being generated.
However, in the prior technique, restrictions on arrangement of the antennas are involved, and there has been a problem in designing a circuit of the cellular phone. Especially, the second antenna disposed in the lower portion of the housing is required to reduce influences caused by covering with the hand. Therefore, further restrictions are imposed on a circuit arrangement.